In humans, expression of the astrocyte marker, glial fibrillary acidic protein (GFAP) increases during aging and neuropathology. Similarly, the expression of apolipoprotein E (ApoE) increases with age in nonhuman primates, in association with amyloid deposition. Because of the potential role of both of these markers in Alzheimer's disease, the relative distributions of GFAP and ApoE were examined in the temporal cortex of the aging rhesus macaque (Macaca mulatta). The distribution of GFAP in the temporal cortex of young and middle-aged adult macaques was similar, with high levels seen in the molecular layers of the dentate granule cells, Ammon's horn and glial limitans. In some older macaques (>25 years), hypertrophic astrocytes could be visualized in the hippocampal pyramidal and deeper layers of the temporal cortex. ApoE expression was only seen in the older monkeys. Although a few plaques were seen scattered in the hippocampus of several of these animals, the majority of the plaques were seen in the temporal cortex. Thus, the more generalized distribution pattern of reactive astrocytes appears to overlap with that of the ApoE plaques, suggesting that a causal relationship might exist between astrogliosis and ApoE deposition. (Findings from this study were presented in 1995 at the Annual Meeting of the Society for Neuroscience, held in San Diego, CA).